1. Field of the Invention
This invention relates generally to a system and method for determining a maximum cell voltage for fuel cells in a fuel cell stack and, more particularly, to a system and method for determining a maximum cell voltage for fuel cells in a fuel cell stack that includes determining the oxidation state of the fuel cell catalyst so that the maximum stack voltage set-point can be adjusted during operation of the fuel cell system to minimize platinum catalyst surface area loss.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte there between. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated at the anode catalyst to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons at the cathode catalyst to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. A PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically, but not always, include finely divided catalytic particles, usually a highly active catalyst such as platinum (Pt) that is typically supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode reactant input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant input gas that flows into the anode side of the stack.
A fuel cell stack typically includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow fields are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow fields are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
It is known that a typical fuel cell stack will have a voltage loss or degradation over the lifetime of the stack. It is believed that the fuel cell stack degradation is, among other things, a result of voltage cycling of the fuel cells in the stack. Voltage cycling occurs when the platinum catalyst particles used to enhance the electro-chemical reaction transition between a low and high potential state, which promotes dissolution of the particles. Dissolution of the particles results in loss of active surface area and performance degradation.
Many factors influence the relative loss in surface area of the platinum particles relating to voltage cycling, including peak stack voltage, temperature, stack humidification, voltage cycling dynamics, etc. Lower stack voltage set-points offer greater protection against degradation, but higher stack voltage set-points provide increased system efficiency. Thus, the control for various fuel cell systems often requires the stack to at least operate at a minimum power level so that, in at least one case, the cell voltages are prevented from rising too high because frequent voltage cycles to high voltage can cause a reduction in the active platinum surface area of the cathode and anode electrodes, as discussed above.
Typically, in known fuel cell systems, a fixed voltage limit is used to set the stack minimum power level to prevent unwanted voltage cycling. For example, a typical voltage suppression strategy may use a fixed voltage set-point, such as 850-900 mV, and prevent the stack voltage from rising above that value. If the fuel cell power controller is not requesting power, or is requesting minimal power, the power generated by the stack necessary to maintain the cell voltage levels at or below the fixed voltage set-point is provided to certain sources where the power is used or dissipated. For example, the excess power may be used to charge a high voltage battery in a fuel cell system vehicle. U.S. Patent Application Publication No. US 2006/014770 A1, published Jul. 6, 2006, titled Reduction of Voltage Loss Caused by Voltage Cycling by Use of A Rechargeable Electric Storage Device, assigned to the assignee of this application and herein incorporated by reference, discloses a fuel cell system that charges a vehicle battery in order to maintain the cell voltage below a predetermined fixed voltage set-point.
If the voltage set-point is relatively high, then the system may often charge the battery, which could cause the battery charge to be at its maximum more often. If the battery is at its maximum charge and cannot except more charging power, then the controller may cause the excess power to be dissipated in other components, such as resistors, in the form of heat to maintain the cell voltage below the maximum voltage set-point, which effects system efficiency as a result of wasting hydrogen fuel.